The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Known spark-ignition (SI) engines introduce an air/fuel mixture into each cylinder which is compressed in a compression stroke and ignited by a spark plug. Known compression ignition engines inject pressurized fuel into a combustion cylinder near top dead center (TDC) of the compression stroke which ignites upon injection. Combustion for both gasoline engines and diesel engines involves premixed or diffusion flames controlled by fluid mechanics.
Spark-ignition (SI) engines can operate in a variety of different combustion modes, including a homogeneous spark-ignition (SI) combustion mode and a stratified-charge spark-ignition (SI) combustion mode. Spark-ignition (SI) engines can be configured to operate in a homogeneous-charge compression-ignition (HCCI) combustion mode, also referred to as controlled auto-ignition (HCCI) combustion, under predetermined speed/load operating conditions. The controlled auto-ignition (HCCI) combustion comprises a distributed, flameless, auto-ignition combustion process that is controlled by oxidation chemistry. An engine operating in the controlled auto-ignition (HCCI) combustion mode has a cylinder charge that is preferably homogeneous in composition, temperature, and residual exhaust gases at intake valve closing time. Controlled auto-ignition (HCCI) combustion is a distributed kinetically-controlled combustion process with the engine operating at a dilute air/fuel mixture, i.e., lean of an air/fuel stoichiometric point, with relatively low peak combustion temperatures, resulting in low NOx emissions. The homogeneous air/fuel mixture minimizes occurrences of rich zones that form smoke and particulate emissions.
When an engine operates in the controlled auto-ignition (HCCI) combustion mode, the engine control comprises lean air/fuel ratio operation with the throttle wide open to minimize engine pumping losses. Air flow into the engine can be controlled by controlling opening and closing of engine intake and exhaust valves, including controlling phasing and lift of opening and closing thereof. When the engine operates in the spark-ignition (SI) combustion mode, the engine control can comprise stoichiometric air/fuel ratio operation, with the throttle valve controlled over a range of positions from 0% to 100% of the wide-open position to control intake air flow to achieve the stoichiometric air/fuel ratio. An engine can have a limited speed/load operating range in the controlled auto-ignition (HCCI) combustion mode due to combustion instability or excessive combustion noise.
It is known to use variable actuation of engine intake and exhaust valves to retain a high proportion of residual combustion products from a previous combustion cycle to provide conditions for auto-ignition in a highly diluted mixture.